Plant genetic engineering has revolutionized agriculture and the way that we use biological systems to generate products for our use. Through transformation and the subsequent regeneration of transgenic plants, a series of agronomically important characteristics or traits have been introduced into domesticated crops. These characteristics or traits include resistance to insects, fungal disease, and other pests and disease-causing agents, tolerance to herbicides, enhanced stability or shelf-life, increased yield, environmental tolerances, and nutritional enhancements.
The success of plant genetic engineering depends on manipulation of gene expression in plants. In one approach, expression of a novel gene that is not normally expressed in a particular plant or plant tissue may confer a desired phenotypic effect. In another approach, transcription of a gene or part of a gene in an antisense orientation may produce a desirable trait by preventing or inhibiting expression of an endogenous gene. The newly introduced genetic elements are collectively referred to as transgenes. A typical transgene comprises, from 5′- to 3′-end, a regulatory sequence, a full or partial coding region in sense or antisense orientation, and often a terminator region. Many variables affect the final expression pattern of the transgene, including, for example, the insertion site of the transgene in the plant genome, the strength and specificity of the regulatory sequence, preferred codon usage in the targeted plant species, and the presence of cryptic splice sites or cryptic poly A sites. However, a reproducible expression pattern of the trans gene is achievable using technologies disclosed herein and elsewhere. The expression pattern of a transgene relates to its being transcribed efficiently at the right time during plant growth and development (temporal expression pattern), in the optimal location in the plant (spatial expression pattern), and in the amount necessary to produce the desired effect. For example, constitutive expression of a gene product may be beneficial in one location of the plant but less beneficial in another part of the plant. In other cases, it may be beneficial to have a gene product produced at a certain developmental stage of the plant or in response to certain environmental or chemical stimuli.
The commercial development of genetically improved germplasm has also advanced to the stage of introducing multiple traits into crop plants, often referred to as a gene stacking approach. In this approach, multiple genes conferring different characteristics of interest can be introduced into a plant. It is important when introducing multiple genes into a plant that each gene is modulated or controlled for the desired expression and that the regulatory elements are diverse in order to reduce the potential of gene silencing. In light of these and other considerations, it is apparent that optimal control of gene expression and regulatory element diversity are important in plant biotechnology.